1. Field of the Invention
The invention relates to the field of electronic devices, and in particular, to transimpedance amplifiers.
2. Background Information
Transimpedance amplifiers are known. Fully balanced or differential transimpedance amplifiers are utilized in a variety of applications where it is desirable to convert a current-varying signal into a voltage-varying signal. One such application is within optical receiver systems where the transimpedance amplifier is used to convert the current-varying output signal of a photodetector into a voltage signal that is processed by other circuitry. Although fully balanced or differential transimpedance amplifiers are typically associated with favorable power supply rejection characteristics, they are also often characterized with a relatively low voltage gain, a large low-cutoff frequency, poor noise performance, limited dynamic range, and/or a low bandwidth.
U.S. Pat. No. 5,343,160, by Stewart S. Taylor, entitled xe2x80x9cFULLY BALANCED TRANSIMPEDANCE AMPLIFIER WITH LOW NOISE AND WIDE BANDWIDTHxe2x80x9d issued Aug. 30, 1994, describes (Abstract) a circuit implementing a fully balanced (differential) transimpedance amplifier with low-noise and wide bandwidth, accomplished by direct coupling the feedback resistors from the outputs of the amplifier to the inputs without the use of a blocking capacitor. An implementation of this transimpedance amplifier device is shown in FIG. 2. As described in the patent, the transimpedance amplifier circuit includes two single-ended input amplifiers that form a first stage and a differential amplifier that forms a second stage common to both input amplifiers. A transistor 210 and a resistor 211 form one input amplifier while transistor 213 and resistor 214 form the other input amplifier. Capacitors 216 and 217 couple the input signal to each input amplifier.
The second stage consists of a differential pair of transistors 220 and 221 and load resistors 222 and 223. Transistors 225, 226 and 227 and diodes 230, 231 and 232 provide proper bias to transistors 220 and 221 with common-mode feedback. This biasing is necessary in some cases to lower the common-mode gain of transistors 220 and 221 since there is an undesired positive common-mode feedback loop around the entire amplifier. Transistors 225 and 226 and diodes 230-232 are biased with current from transistor 235. Transistors 210 and 213 are biased with common-mode feedback from transistors 220, 221, 237, diodes 238 and 239, and resistors 241 and 242. Biasing current is provided by resistors 245 and 247 and transistor 246. Resistors 241 and 242 and capacitors 216 and 217 are chosen consistent with the low frequency cutoff of the circuit and with the size limitations of the integrated circuit. Resistors 241 and 242 should be much larger than feedback resistors 260 and 261 to ensure good noise performance.
The photodetector (MSM in this case) is modeled by current source 250 and capacitor 251. The voltage offset means are implemented with source follower transistors 255 and 256 and level-shifting diodes 257, 258 and 259. Transistors 256 and 285 provide bias current to level-shifting diodes 257-259. Feedback resistors 260 and 261 couple the offset output signals back to the inputs of the photodetector. The output can be taken from the sources or gates of transistors 255 and 256, or from the offset means diodes 257, 258 and 259.
However, a least one drawback of the arrangement described in the patent is that it uses two input capacitors (216, 217) which disadvantageously requires four times as much capacitor area. This is clearly a disadvantage because such space is at a premium in integrated circuitry. Another drawback with the U.S. Pat. No. 5,343,160 patented transimpedance amplifier is that it requires compensation to achieve stability.
Therefore, a need exists for an amplifier which allows for single integrated capacitor coupling, for high bandwidth, low noise, low voltage operation with an integrated detector, that does not have the disadvantages of existing circuits.
It is, therefore, a principle object of this invention to provide a fully balanced transimpedance amplifier for high speed and low voltage applications.
It is another object of the invention to provide a fully balanced transimpedance amplifier for high speed and low voltage applications that solves the above mentioned problems so that single integrated capacitor coupling, for high bandwidth, low noise, low voltage operation with an integrated detector is achieved.
These and other objects of the present invention are accomplished by the method and apparatus disclosed herein.
According to an aspect of the invention, a fully balanced transimpedance amplifier for high speed and low voltage applications is provided.
According to an aspect of the invention, an amplifier which allows for single integrated capacitor coupling, for high bandwidth, low noise, low voltage operation with an integrated detector is provided.
According to another aspect of the invention, an input stage of the amplifier uses a matched pair of common source connected transistors with sources tied directly to ground to eliminate the Vds overhead usually found in differential pairs. Ground connection minimizes a source resistance noise component, while matching minimizes power supply noise generation and susceptibility for an array of amplifiers.
According to another aspect of the invention, feedback resistors along with diode connected MESFETS determine the transimpedance of the amplifier. The nonlinearity of diodes helps to soften clipping. Transresistance also determines the noise generated by the amplifier. Diode connected MESFETS offer lower noise than resistors for the same impedance.
According to another aspect of the invention, stability is achieved through use of only a single stage of gain in a loop of the input stage. Therefore, in contrast with the patented transimpedance amplifier referred to in the Background section above (U.S. Pat. No. 5,343,160), no additional compensation is required to achieve stability.
According to another aspect of the invention, additional gain is achieved through cascading in the input stage.
According to another aspect of the invention, a differential stage minimizes any difference in amplitude between two sides of the amplifier input stage.
According to another aspect of the invention, two stages of source followers provide buffering to drive a relatively low impedance load, e.g., a 50 Ohm load at the output of the amplifier.
These and other aspects of the invention will become apparent from the detailed description set forth below.